The present invention relates to wavelength division multiplex (WDM) optical communication, and more especially to an optical cross-connect (OXC) for use in a WDM optical communication network.
As is known, WDM optical communication network comprise a plurality of spatially disposed nodes which are interconnected by optical fibre waveguides in a network configuration. Networks are commonly configured as rings in which the nodes are interconnected in serial manner to form a closed loop or ring. Communication traffic is communicated between the nodes by optical radiation modulated by the communication traffic which is conveyed by the optical fibres.
Optical radiation in the context of the present patent application is defined as electromagnetic radiation having a free-space wavelength of 500 nm to 3000 nm, although a free-space of 1530 nm to 1570 nm is a preferred part of this range. In wavelength division multiplexing, the optical radiation is partitioned into a plurality of discrete non-overlapping wavebands, termed wavelength channels, or optical channels, and each wavelength channel is modulated by a respective communication traffic channel.
As is known the network nodes often include an optical add drop multiplexer (OADM) for adding/dropping selected wavelength channels to the network to thereby establish routing of communication traffic channels between nodes in dependence upon the carrier wavelength of the wavelength channel. In order to be able to selectively route (cross-connect) communication traffic between respective parts of the communication network, such as for example routing of communication traffic between interconnected rings of the network, requires the node at the interconnection of such network parts to include an optical switching capability. Such optical switching arrangements are termed optical cross-connects (OXCs) and can be broadly classified as those which are (i) non-wavelength selective and capable only of switching all WDM wavelength channels appearing at a given input fibre to a selected output fibre and consequently referred to as fibre cross-connects (FXCs), and (ii) those capable of wavelength cross-connection (interchange) which are able to cross-connect selected wavelength channels from one a given input to a selected optical output. In the case of the latter it is often desirable for the OXC to be capable of additionally adding one or more selected wavelength channel/s to the network via a selected output/s and dropping (terminating) one or more selected wavelength channel/s from the network via a selected input/s.
In this patent application an optical cross-connect (OXC) is defined as an optical switching arrangement in which all switching takes place in the optical domain. This is to be contrasted to switching arrangements, sometimes also referred to as being optical on account of them having optical inputs and outputs, in which the optical input radiation is converted to an electrical signal for switching before being converted back optical radiation.
In its simplest case, an optical switching arrangement can be regarded as an optical switching matrix in which the inputs form rows of the matrix and the outputs form columns of the matrix. At each crossing point between an input and an output, there is an optical switching element which can be selectively closed in order to connect an input to selected output.
A switching matrix which is always capable of connecting any given input to a desired output, regardless of existing connections in the matrix, is termed non-blocking. The size of a non-blocking switching matrix is determined by the product of the numbers of input and outputs, i.e. it increases quadratically with the number of connections which are required to be simultaneously established. For example, a non-blocking OXC having M optical inputs and M optical outputs each capable of supporting N wavelength channels requires an optical switching matrix of size (M×N)×(M×N) which for an 8 input/output, 80 wavelength channel OXC requires an optical switching matrix which is of size at least 640×640. In addition, where it is required for the OXC to be able to add/drop one or more wavelength channels, this requires the switching matrix to be correspondingly larger. OXCs which utilise a single optical switching matrix of this size are, with current technology, expensive and hard to develop. Furthermore, if the connection capacity of such a switching matrix no longer meets current demands, it has to be replaced, further increasing the cost of the communication system. An advantage of an OXC having a single switching matrix is that it is non-blocking and has a low insertion loss since there is only a single switching stage in the through path between any optical input and any optical output as well as only a single switching stage on the add/drop path.
To reduce the size of the switching matrix the optical cross-connect shown in FIG. 1 has been proposed which includes a respective smaller sized switching matrix for each wavelength channel. As will be appreciated, this OXC architecture is still single stage in that all through connections and adding/dropping of wavelength channels involves traversing a single one of the switching matrices. Referring to FIG. 1, the OXC comprises a plurality M of optical inputs and a plurality M of optical outputs, denoted I1 to IM and O1 to OM respectively (typically the inputs and outputs comprise an optical fibre). Each of the inputs is able to receive WDM radiation comprising a plurality N of wavelength channels of carrier wavelengths λ1 to λN. Thus the OXC has the capability for cross-connecting M×N communication channels.
Each optical input I1 to IM is connected to an input of a respective wavelength de-multiplexer D1 to DM. Each de-multiplexer, which has N outputs, spatially separates the WDM radiation appearing at its input such that a respective one of the wavelength channels appears at a respective output of the de-multiplexer.
The OXC further comprises a plurality N (one for each wavelength carrier) of switching matrices S1 to SN. Each switching matrix has at least M inputs and M outputs. (In the example illustrated in FIG. 1 the switching matrices each have M+2 inputs and outputs enabling the OXC to additionally add/drop up to two of each wavelength carriers.) A switching matrix is assigned to a respective carrier wavelength λ1 to λN. In the example, the switching matrix S1 is assigned for switching only communication channel having a carrier wavelength λ1, S2 is for switching only communication channels having a carrier wavelength λ2, . . . , and SN is for switching only communication channels having a carrier wavelength λN. Assignment of the switching matrices in this way is achieved by connecting the output of each of the M de-multiplexers corresponding to a given wavelength carrier, to a respective one of the inputs of the switching matrix assigned to that wavelength carrier.
Each output of each switching matrix is connected to a corresponding input of one of M multiplexers M1 to MM, which receives wavelengths λ1 to λN from the various switching matrices S1 to SN at its N inputs and multiplexes these to the output O1 to OM, respectively. In order to route a communication channel correctly through the OXC, it is sufficient to supply it to the multiplexer which is connected to the required output. The input of this multiplexer at which the communication channel arrives is defined by its carrier wavelength.
The OXC of FIG. 1, in common with the OXC having a single switching matrix, has the benefit of a low insertion loss and has the further benefit that it can be upgraded when additional wavelength channels are subsequently added to the communication system. Upgrading is achieved by adding a further switching matrix for each additional wavelength channel and by increasing the number of outputs of the de-multiplexers and inputs of the multiplexers. Existing switching matrices can continue to be used without modification. Thus it is possible to build up a telecommunication network with little initial investment corresponding to the required capacity and to upgrade it according to demand.
There exists, however, a problem with adding or dropping wavelength channels with the OXC of FIG. 1. In order to be able to terminate a number A of wavelength channels without blocking, each switching matrix S1 to SN must additionally include A inputs and outputs. If the demand for dropping wavelength channels increases, this can only be satisfied by either re-assigning inputs and outputs of the switching matrices at the expense of the through-traffic (whereby the number of wavelength channels useable on the input and output is decreased), or by replacing each of the switching matrices by ones having a higher number of inputs and outputs. In the latter case, existing switching matrices can no longer be used when upgrading the OXC and the cost for an upgrade is considerably increased.
The present invention arose in an endeavour to provide an OXC which is capable of adding/dropping selected wavelength channels and whose structure is capable of being adapted to increase the number of wavelength channels that can be added/dropped whilst continuing to use existing components.